The structure of a repeated DNA sequence that is expressed during development of Xenopus laevis has been studied. A conserved repeat unit of about 3.5 kb is flanked by clusters of tandemly repeated sequences with a 180 bp period. To study genes that are first expressed early in development we have prepared a cDNA library highly enriched for sequences expressed in gastrula but rare or absent in unfertilized eggs. We have isolated a cDNA clone derived from mRNA for the ubiquitous Ca-binding protein calmodulin; the sequence of this cDNA has been determined.